Encapsulation, and particularly micro-encapsulation, of various materials is a common means of providing such materials with a protective environment. Encapsulation with, for example, a urea-formaldehyde resin, provides such materials with an external matrix which insulates them--partially or completely--from conditions which might lead to their loss or to loss of their desired, active properties.
Typical examples of the use of encapsulation include protection of pigments or dyes to obtain improved bleed resistance, light fastness and compatibility. Water-soluble fire retardants are commonly encapsulated for use under conditions of high humidity. Pharmaceuticals, pesticides, herbicides, anti-fouling agents, catalysts and the like may be encapsulated in order to provide for controlled release of such materials over an extended period of time. Similarly, encapsulation of photographic emulsions provides improved characteristics; of opacifying agents such as titanium dioxide, for more efficient pigment particle spacing; and of other well known materials for improvement in their particular uses.
The specific methods of encapsulating these various materials vary widely, even where, for example a specific encapsulating material such as urea-formaldehyde prepolymer is utilized. Commonly, however, their basic approach includes the steps of:
A. forming an admixture of the material, emulsifier and prepolymer in a biphasic, liquid system; PA1 B. emulsifying the admixture to form fine droplets containing the material; PA1 C. curing the prepolymer about the dispersed droplets of material; and PA1 D. separating the resultant, solid, encapsulated material from remaining liquid.
In general, these prior art methods suffer from one or more of a number of basic procedural drawbacks. Chief among them is the approach to curing of the encapsulating prepolymer.
In some procedures, strong acid catalysts have been added directly to the aqueous phase containing a water dispersible material. This addition is commonly performed even prior to admixture of the aqueous phase with the water-immiscible phase. In such procedures, polymerization commences early in the encapsulation process. This greatly limits the time for reaching a desired particle size because curing or gellation occurs too rapidly.
In other procedures, where a latent acid catalyst is added directly to the aqueous phase, the oil and water emulsion must be heated to activated the catalyst. This often leads to undesirable changes in particle size and even to a phase separation between the two liquids.
In addition, many water-dispersible materials flocculate under acidic or similar encapsulation conditions. The process consequently becomes difficult to control because the particle size of a material may grow unduly large prior to encapsulation and/or the material may lose its water-dispersible characteristic before encapsulation can be performed.